There have been many attempts to produce electrically conductive coatings and these have generally fallen into one of two types. One type has been formed from a paint having a pigmentation of metal particles while the other type has been formed from compositions containing carbon or graphite.
The paints having the pigmentation of metal particles give coatings having a very low resistivity and their purpose has been to provide conducting coatings having a high conductivity.
The mixtures containing carbon and graphite, on the other hand have had much higher resistivities.
Thus, when applied as thin layers, e.g. 0.001 to 0.005 inch in thickness, they have quite high resistivities e.g. 100 ohms per square or much more, while if applied as thicker coatings there would be problems of cracking of the films with the passage of time and these thicker coatings could not be applied in the simple way by paintng.
There is need for electrically conductive coatings which have a resistivity such that they can be used as heating elements, for example, when applied to the walls of a building. Thus, the paints having the pigmentation of metal particles would be completely unsuitable because of their high conductivity since excessively high currents would be required to give reasonable heat outputs. The mixtures containing carbon or graphite on the other hand have too high a resistivity and to give reasonable heat outputs they would require too high a voltage drop for safety. Thus, for use on the wall of a building the total voltage drop across the wall should not excess the 40 volt limit defined as low voltage by the Institution of Electrical Engineers (United Kingdom).
It is, therefore, an object of this invention to provide a paint composition which has a paint-like consistency and so can readily be spread into the form of thin coatings and yet which has a low resistivity, e.g. from 0.1 to 10 ohms/square, so that thin coatings of it give a reasonable heat output at low voltages.